Digital to analog converter



June 27, 1967 J. J. STONE ETAL` DIGITAL TO ANALOG CONVERTER Filed Nov. 19, 1965 4|' 7| 92 94 |ME05| f i N CHAN 6| i l/ 2 MEG |O24tl E m--Mf--e IK 52 CHAN 2'L 2 5ml 2.2 IME@ 1 CHAN 3 25 ,MEG 54 14 74 256 Q6 1 l C||AN|L K 545 I uT|L|zAT|oN 4 I MEG 98 D|= \/|CE 'K |ME6 55 45 28a 55 5 CHAN 25 64A |ME 56 lr-Ml 56 CHAN E 52a 6 wie 57 CHAN 7K K 37 6 D.. 27a I mi@ 58 48 --v 3a CHANd I A an 28 I MAE@ 59 9 l K 59 CHAN 9,L I 4m 29 MAE@ 60 CHAN@ C 'l j'S| 1n |000 v POWER V SUPPLY H 0 C Arlu I m l /osfpf/J STONE I J @OMA/v A. ADAMS f f' 1 I INVENTORS p Ems OTENUALN O j A #ORA/Ey United States Patent O 3,328,792 DIGITAL T ANALOG CONVERTER Joseph James Stone, Glenview, and Roman A. Adams, Skokie, Ill., assgnors to A. B. Dick Company, Chicago, Ill., a corporation of Illinois Filed Nov. 19, 1963, Ser. No. 324,630 4 Claims. (Cl. 340-347) This invention relates to digital to analog converters and more particularly, to improvements therein.

An object of this invention is to provide a novel digital to analog converter circuit.

Another object of this invention is the provision of an accurate and stable digital to analog converter circuit.

Still another object of this invention is to provide an accurate and stable digital to analog converter which is relatively inexpensive.

rl`hese, and other objects of this invention, may be achieved by the provision of a circuit wherein a relatively stable power supply applies current to a plurality of gating networks. A resistive ladder network has tapping points therealong each of which is connected through a different one of the gating networks back to the power supply. One end of the resistive ladder is connected to the power supply and the other end to the summing point or junction of an operational amplifier. In the presence of an input signal to one of the gates, current is caused to flow through the ladder network .in a manner such that the summing junction at one end of the ladder network receives current having an amplitude proportional to the tapping point on the ladder network connected to the gate which has received the input signal. A separate input terminal is provided for each digital signal and the summing junction receives an output current whose amplitude is established lby the terminal to which the input signal is applied.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which the drawing is a circuit diagram of an embodiment of the invention.

Referring now to the drawing, which is a circuit diagram of an embodiment of the invention, it will be seen that provision is made for ten digital inputs respectively labeled Channel 1 through Channel 10. A single power .supply 12 is employed. It should be noted at this point that values of circuit components which were employed are shown on the drawing. These are shown by way of example, and are not to be construed as a limitation upon the invention.

Each input terminal from Channel 1 through Channel is respectively connected through a load resistor respectively 21 through 30 to the positive end or ground side of the power supply 12. Each of Channels 1 through 10 lis also respectively connected through a diode respectively 31 to 40 to a junction point respectively 41 through 50.

Each one of the junction points respectively 41 through 50 is connected through a separate bleeder resistor, respectively 51 through 60 to the negative terminal of the power supply 12.

ice

A ladder network is provided, comprising a series connected string of resistors respectively 61 through 70. One end of the resistor ladder network is connected to the ground, or positive side, of the power supply 12. The other end of the ladder network 92 is connected through a diode 71 to the junction point 41. The junctions between the successive resistors in the series string are respectively connected by the respective diodes 72 through 80 to the respective junction points 42 through 50. The top of the ladder network is also connected to a summing junction 92. An operational amplifier 94 is connected to the summing junction. Its output is used to drive any desired utilization device 96. Eectively, the resistive ladder is an input resistor for the operational amplifier.

It should be noted that the values of the resistors selected for the ladder network commencing with resistor 61 through resistor 69 are such that each succeeding resistor has a value one-half that of its preceding adjacent resistor. Thus, resistor 61 has a value of 1024 ohms and resistor 62 has a value of 512 ohms. Resistor 69 has a value of 4 ohms as does also resistor 70. These values are chosen such that the value of current which will ilow into the summing junction 92 will vary .in a binary progression as the channel input associated therewith has a signal applied thereto. The ladder resistors, however, can have their values adjusted to provide any type of progression desired.

A trimming resistor 98 serves the function of trimming the current iiow through the ladder network so that precise operation is obtained.

Initially assume that a slightly positive bias potential is applied to all channel input terminals (this need only exceed one volt), from bias potential source 100. As a result, diodes 31 through 40 are biased conductive. There will then be a parallel current iiow from the power supply through resistors 21 through 30, through junction points 41 through 50, through resistors 51 through 60 back to the power supply. This current through each path is substantially one milliampere in amplitude. The junction points are all suiiiciently positive to bias off diodes 71 through S0 so that there is no current flow through the resistive ladder network.

Assume now that a digital signal is applied to one of the channel input terminals which is suiiiciently negative (here, too, it need only exceed one volt negative) to bias off the one of the diodes connected to that channel input terminal. For example, assume that the negative input signal is applied to the channel two input. Diode 32 is biased off by the signal. This causes a switching of the current which ilowed through diode 32 to a path which may be traced from the power supply through the one megohm resistor 52, through junction 42 and through diode 72 to the tap on the resistive ladder network. Here there is a current division, part of the current returning to the power supply through the ladder network connected directly back to the power supply and the other part returning by way of the summing junction 92 in the input to the operational amplifier 94.

As is well known, an operational amplifier operates in such a manner as to maintain the potential of itssumming junction equal to zero. Thus, it may be described as being at a virtual ground potential. Current applied to a junction of the ladder network, such as the one between resistors 61 and 62 to which channel 2 input is coupled, will divide between the actual Iground and virtual ground (summing junction 92) in proportion to the resistance values in eachV leg lof the ladder network. In accordance with this invention, the ladder network is made an input resistor connecting to the summing junction of the operational amplifier.

Thus, a negative voltage signal applied to channel 1 causes one milliampere of current to flow into the summing point. A negative voltage signal applied to channel 2 causes one-half milliampere of current to flow into the summing point. A negative signal applied to channel 3 will result in onequarter millampere of current being delivered to the summing point.

As each one of the channels in turn has a negative input signal lapplied thereto, the value 'of the current delivered at the summing point changes in accordance with progression. When a negative input signal is Iapplied to channel 10, the diode 40 is blocked and the current delivered t-o the summing point will have a value of 1612 milliamperes. The operational amplier can then drive the utilization device 96 with an output signal which is an analog representation of the channel from which the digital signal was received.

The ladder resistors 61 through 70 can be adjusted for any type of progression, for instance, decimal. The polarity of .the bleeder current can be reversed, along with the polarity of the switching diodes, and the direction of current c-an be reversed at the summing point to provide negative outputs. A combination of both polarities of bleeder currents can be used with the ladder extended from the summing point in the opposite direction for complementary operation. Analog inputs can also be handled where the bleeder resistance to the summing point from the input is adjusted for full scale analog voltage input, and of either polarity.

The arrangement for a digital to analog computer shown in the drawing provides a variation of weighting in the `ratio of 512 to 1. The ladder network provides a means for obtaining an accurate current weighting. The accuracy of the entire system is associated with the stability land regulation of the power supply and the resistance accuracy of the bleeder and ladder network resistors. It is desirable to have a high ratio of bleeder resistance value to total ladder network resistance value. The circuit shown :has been built and operates to convert digital inputs to corresponding current levels at the summing point at rates up t-o one megacycle.

There has accordingly been described 'and shown herein a novel, useful, accurate and relatively inexpensive analog to digital converter arrangement. It will be appreciated that, while the invention has been exemplified by showing ten channels of digital input being converted to a single analog output, fewer than, or more than, this number of channels may be handled without departing from the spirit and scope of this invention.

We claim:

1. A digital to analog lconverter circuit comprising an operational amplifier having an input summing junction, a ladder network connected to said input summing junction, said ladder network comprising a plurality of series connected resistors, a plurality of digital input terminals, a plurality of coupling means a different one of which couples a different one of said digital input terminals to a different one of the junctions between a different two of series connected resistors, each said coupling means including two serially connected, relatively oppositely poled diodes connected between a digital input terminal and a junction between two resistors, a irst load resistor, and a current setting resistor, and a power supply having a first and second output terminal, means coupling each said rst load resistor between a different digital input terminal and said first output terminal, `and means coupling each said current setting resistor between each serially connected relatively oppositely poled diodes and said second output terminal, means for applying a signal to a predetermined one of said digital input terminals, and means within each of said coupling means responsive to the application of a signal to the digital input terminal coupled thereto for applying a predetermined current to the junction coupled thereby to cause a value of current to flow through said summing junction as determined by the resistance value of the series connected resistors -on either side of the junction of resistors to which said predetermined `current is applied.

2. A digital to anolog converter circuit comprising an operational amplifier havin-g an input summing junction, a ladder network connected to said input summing junction, said ladder network comprising a plurality of series connected resistors, a plurality of digital input terminals, a plurality of coupling means a diierent one of which couples a different one of said digital input terminals to a different one of the junctions between a diiferent two 0f said series connected resistors, means for applying a signal to a predetermined one of said digital input terminals, means within each of said coupling means for establishing a first current ow path which excludes the junction between two resistors to which said coupling means is coupled, and a second current flow path which includes said junction between said two resistors, means within each of said coupling means for establishing a predetermined current flow within said rst current flow path, and means within each of said coupling means responsive to the application of a signal to the digital input coupled thereto for transferring said predetermined current ow to the second current path including the junction included therein to cause a value of current to ow through said summing junction as determined by the resistance value of the series connected resist-ors on either side of the junction of `resistors to which said predetermined current is applied.

3. A digital to anolog converter network comprising a plurality of digital input terminals, an analog output terminal, a ladder network comprising a plurality of series connected resistors, there being as many resistors in said plurality as there are digital input terminals, a power supply having a -pair of output terminals, means connecting one of said power supply output terminals to one end of said ladder network, summing junction means coupled to said one of said output terminals, means coupling the other end `of said ladder network to said summing junction, a plurality of load resistors, a different load resistor being connected between each one of said input terminals and said one of said power supply output terminals, means coupling the junction between two adjacent resistors in said resistive ladder network to the other terminal of said power supply including rst diode means connected in series with a resistance, and a separate, second diode means coupled between each input terminal and the junction between the first diode means and resistance connected to said power supply, the polarity of the coupling ofthe respective rst and second diode means to one `another being relatively opposite.

4. A digital to analog convertei comprising a plurality of digital input terminals, an analog output terminal, `a ladder network, a power supply having a first and second output terminal, means connecting said power supply rst output terminal to one end of said ladder network, means connecting the other end of said ladder network to said analog output terminal, means coupling said analog output terminal to said power supply first output terminal, said ladder network comprising a plurality of separate resistors connected in series, there bein-g .as many resistors in said plurality as there are digital input terminals, each of said resistors being associated with a different one of said digital input terminals and having its value selected in accordance with the weighting of the analog output to be associated with the digital input terminal,- means coupling each one of said digital input terminals to one end of the associated resistance in said ladder network, said means including a first diode means connected 5 6 in series with a second diode means, said series connected References Cited diode means being poled relatively oppositely to one an- UNITED STATES PATENTS other and connecting to each other `et a junction, a different bleeder resistor connected between each junction 3182302 5/1965 Hom 340"347 and the second output terminal of said power supply, each 5 OTHER REFERENCES said bleeder resistor having a resistance value which is IBM Tech Bisel, Bulletin, volume 3, NO 7, Decem.

large relative to the resistance value of said resistive ladber 196()J p 17- der network, and a separate load resistor connected bel, tween each one of said input terminals ,and the rst out- MAYNARD R' WILBUR P'lmary Examme" put terminal of said power supply. 10 K. R. STEVENS, J. WALLACE, Assistant Examiners. 

1. A DIGITAL TO ANALOG CONVERTER CIRCUIT COMPRISING AN OPERATIONAL AMPLIFIER HAVING AN INPUT SUMMING JUNCTION, A LADDER NETWORK CONNECTED TO SAID INPUT SUMMING JUNCTION, SAID LADDER NETWORK COMPRISING A PLURALITY OF SERIES CONNECTED RESISTORS, A PLURALITY OF DIGITAL INPUT TERMINALS, A PLURALITY OF COUPLING MEANS A DIFFERENT ONE OF WHICH COUPLES A DIFFERENT ONE OF SAID DIGITAL INPUT TERMINALS TO A DIFFERENT ONE OF THE JUNCTIONS BETWEEN A DIFFERENT TWO OF SERIES CONNECTED RESISTORS, EACH SAID COUPLING MEANS INCLUDING TWO SERIALLY CONNECTED, RELATIVELY OPPOSITELY POLED DIODES CONNECTED BETWEEN A DIGITAL INPUT TERMINAL AND A JUNCTION BETWEEN TWO RESISTORS, A FIRST LOAD RESISTOR, AND A CURRENT SETTING RESISTOR, AND A POWER SUPPLY HAVING A FIRST AND SECOND OUTPUT TERMINAL, MEANS COUPLING EACH SAID FIRST LOAD RESISTOR BETWEEN A DIFFERENT DIGITAL INPUT TERMINAL AND SAID FIRST OUTPUT TERMINAL, AND MEANS COUPLING EACH SAID CURRENT SETTING RESISTOR BETWEEN EACH SERIALLY CONNECTED RELATIVELY OPPOSITELY POLED DIODES AND SAID SECOND OUTPUT TERMINAL, MEANS FOR APPLYING A SIGNAL TO A PREDETERMINED ONE OF SAID DIGITAL INPUT TERMINALS, AND MEANS WITHIN EACH OF SAID COUPLING MEANS RESPONSIVE TO THE APPLICATION OF A SIGNAL TO THE DIGITAL INPUT TERMINAL COUPLED THERETO FOR APPLYING A PREDETERMINED CURRENT TO THE JUNCTION COUPLED THEREBY TO CAUSE A VALUE OF CURRENT TO FLOW THROUGH SAID SUMMING JUNCTION AS DETERMINED BY THE RESISTANCE VALUE OF THE SERIES CONNECTED RESISTORS ON EITHER SIDE OF THE JUNCTION OF RESISTORS TO WHICH SAID PREDETERMINED CURRENT IS APPLIED. 